A current source is a very high output impedance electrical element which provides a predetermined (typically fixed) output current. The magnitude of the output current is substantially independent of both the voltage impressed across the current source and the load impedance presented to the output of the current source. In so-called "controlled" current sources, the magnitude of the output current, rather than being fixed, is a function of a control signal or a selected circuit parameter, such as a resistor value.
A current mirror circuit is a particular type of controlled current source in which the output current is controlled by the input current applied to the current mirror circuit. Current mirror circuits known in the art typically include first and second base-coupled transistors. The input or controlling current is extended to the collector of the first base-coupled transistor while the output or controlled current is obtained from the collector of the second base-coupled transistor. Base drive current for both the first and second base-coupled transistors is provided by control summing circuitry which draws a small amount of drive current from the bases of the first and second base-coupled transistors and inserts this drive current into the collector lead of one or the other of the base-coupled transistors.
Ideally, the input and output currents of a unity-gain current mirror circuit should have identical magnitudes. In practice, however, some input/output current deviation is always encountered due to the fact that the drive current for the current mirror circuit is diverted from the bases of the base-coupled transistors by the control summing circuit to the collector of one of these base-coupled transistors. In some current mirror circuits this deviation is as small as ##EQU1## per unit of input current, wherein beta (beta) is the common-emitter current gain of the transistors comprising the current mirror circuit. In some applications, however, more precise input/output current matching may be required. Moreover, some applications may require a current mirror circuit having higher output impedance than is typically provided by these arrangements.
A step toward solving this current deviation problem is disclosed in U.S. Pat. Nos. 3,936,725 and 4,166,971 issued Feb. 3, 1976 and Sept. 4, 1979, respectively, to the above-named applicant. In these patents, added precision is achieved by cascading current mirror circuits to produce a square array containing at least three columns of serial-controlled semiconductor devices. Currents flowing through the various controlled semiconductor devices from one current mirror circuit to the next are essentially confined to flow in the respective columns of such serial-controlled devices.
Each current mirror circuit in this arrangement is a row of the square array and is provided with a base drive circuit responsive to the signal at the collector of at least one controlled transistor in the current mirror circuit. Such base drive-collector connections are distributed throughout the array structure on a one or more per row basis in accordance with a connection scheme which restricts each column of serial-controlled semiconductor devices to contain exactly the same number of base drive connections as does every other column.